Not Applicable
Not Applicable
This invention relates to video and multimedia projectors and more particularly to a patterned-silvered mirror for propagating illumination toward a micro-electromechanical display device (xe2x80x9cMDDxe2x80x9d) and reflecting image bearing light rays emanating from the MDD toward a projection lens.
Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction. Such multimedia projection systems typically receive from a personal computer (xe2x80x9cPCxe2x80x9d) analog video signals representing still, partial-, or full-motion display images that are converted into digital video signals for controlling a digitally driven imageforming device, such as an MDD, a common type of which is a digital micromirror device. MDD-based projectors are popular because the MDD is a very efficient, albeit expensive, display device. Accordingly, MDDs are typically employed in single light path, frame sequential color projector configurations. An example of such a projector is the model LP130 manufactured by In Focus Corporation, Wilsonville, Oreg., the assignee of this application.
Significant effort has been invested into developing light-weight, portable multimedia projectors that produce bright, high-quality, color images. However, the weight, size, and optical performance of such projectors is often less than satisfactory. For example, suitable projected images having adequate brightness are difficult to achieve, especially when using compact portable color projectors in a well-lighted room.
FIG. 1, shows a typical prior art multimedia projector 30 in which a light source 32 propagates polychromatic light along an optical path 34 including a condenser lens 40, a color wheel 42, a light integrating tunnel 44, a planar fold mirror 46, a relay lens 48, an MDD 50, and a projection lens 52. One or two field lenses (not shown) typically follow light integrating tunnel 44. A display controller 56 receives color image data from a PC 58 and processes the image data into frame sequential red, green, and blue image data, sequential frames of which are conveyed to MDD 50 in proper synchronism with the rotating angular position of color wheel 42. Display controller 56 controls MDD 50 such that light propagating from relay lens 48 is selectively reflected by MDD pixel mirrors either toward projection lens 52 or toward a light-absorbing surface 66.
To achieve adequate projected image brightness and uniformity, light integrating tunnel 44 collects light exiting color wheel 42 and homogenizes the light during propagation through tunnel 44 to an output aperture 72. The uniformly bright rectangular light bundle exiting output aperture 72 propagates through the field lenses, reflects off fold mirror 46, and is imaged by relay lens 48 onto MDD 50.
Other workers have tried making simpler single path MDD-based projectors. For example, U.S. Pat. No. 6,129,437 for IMAGE DISPLAY APPARATUS describes a similar MDD-based projector in which a concave mirror combines the functions of planar fold mirror 46 and relay lens 48 to simplify the optical path of the projector. While this is an improvement over other optical path configurations, employing mirrors in an MDD light path is not without its problems.
FIG. 2 reveals a source of such problems. An MDD 76 includes an array of micromirrors that each pivot about a hinge axis 78 that, in this embodiment, is parallel to an edge margin of MDD 76. MDD 76 receives an incident light bundle 80 and reflects a reflected light bundle 82, the centers of which are separated by an angle 84 corresponding to the mirror tilt angle range of MDD 76. To achieve the maximum possible projected brightness, incident light bundle 80 and reflected light bundle 82 should each have a low f/#, which results in bundles 80 and 82 almost touching at their closest points. This means that the optical components separating incident light bundle 80 from reflected light bundle 82 must have a sharp cutoff to prevent unwanted spillover of incident light into the reflected light bundle. Such light spillover causes a reduction in contrast ratio of the projected display.
There are several reasons why mirrors are disadvantageous for separating incident light bundles from reflected light bundles in an MDD-based projector. The mirror edge margins are often carefully shaped and positioned to reflect one light bundle while not blocking the other light bundle. The edge shaping is often a curved contour shaped to accommodate the light bundle shapes, projection lens barrel, and folded light paths typically found in compact projectors. Planar mirrors are typically shaped by a xe2x80x9cscribe and breakxe2x80x9d process, which is unreliable for curved breaks. Concave mirrors may also be aspherical, and are shaped by expensive grinding and polishing processes. Both planar and concave mirrors typically have an extra edge margin to accommodate the larger tolerance of the manufacturing processes. Clearly, these manufacturing and adjustment processes work against providing a sharp cutoff between the incident and reflected light bundles.
What is still needed, therefore, is a means of simplifying the optical path of a light-weight, portable projector without reducing the projected image contrast ratio.
An object of this invention is, therefore, to provide an apparatus and a method for improving the compactness, brightness, and contrast ratio of a MDD-based multimedia projector.
Another object of this invention is to provide improved mirrors for use in MDD-based multimedia projectors.
A first preferred embodiment of this invention provides a multimedia projector including a light source for propagating intense illumination through a color modulator. Light exiting the color modulator enters an input aperture of a light integrating tunnel. The light propagates by multiple internal reflection through the light integrating tunnel and exits through an output aperture. Field lenses image light from the output aperture through a transparent portion of a patterned-silvered mirror, through an optional field lens, and onto the micromirror array in an MDD. Micromirrors in the MDD that are tilted to an image-forming angle, reflect the image-forming light back toward a reflective portion of the patterned-silvered mirror, which reflects the image-forming light through a projection lens. In this embodiment, a linear boundary separates the transparent and reflective portions of the patterned-silvered mirror. The reflective portion is formed by depositing a metallic or dielectric coating, which is accurately positioned by masking and formed by conventional processes. This eliminates the need for either an edge-grinding process or a scribe-and-break process and results in a less costly mirror having a very sharp, well controlled boundary. The sharp cutoff prevents or reduces unwanted spillover of an incident light bundle into a reflected light bundle.
A second preferred embodiment of this provides a multimedia projector including a light source for propagating intense illumination through a color modulator and light integrating tunnel as before, but is configured for off-axis illumination of an MDD having an array of diagonally hinged micromirrors. Field lenses image the light from the light integrating tunnel by reflection off a reflective portion another patterned-silvered mirror, through the optional field lens, and onto the MDD. Micromirrors in the MDD that are tilted to an image-forming angle, reflect the image-forming light back through the optional field lens toward a transparent portion of the patterned-silvered mirror, which propagates the image-forming light through the projection lens. In this embodiment, a nonlinear boundary separates the reflective and transparent portions and further includes portions non-parallel to an edge margin of the patterned-silvered mirror. Moreover, the boundary may follow a predetermined curvature to simplify mirror mounting, improve the cutoff between the incident and reflected light bundles, and conform to the geometric shapes of the light bundles.
The patterned-silvered mirrors of this invention are less costly, easier to manufacture, mount, and adjust, and provide a higher contrast projected image by reducing the amount of spillover, scattered, and flat state light entering the projection lens.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.